1. Field of the Invention
The present invention relates to an autofocus adjustment apparatus for a camera and to an autofocus adjustment apparatus having an interchangeable lens.
2. Description of the Related Art
Advanced cameras are available with autofocus adjustment devices which simplify operations for a camera user. These autofocus adjustment devices are provided with camera systems having a movable focusing lens as part of a "photographic optical system," wherein the focusing lens can be adjusted to provide optimal focus. Autofocus adjustment devices typically detect a defocus amount of a subject image plane with respect to a predetermined focal plane of the photographic optical system, convert this defocus quantity into a lens drive amount and move the focusing lens by this lens drive amount.
With these types of focus adjustment devices, a conversion factor for converting the defocus quantity into a lens drive amount is precalculated and stored for each lens position. When it is required to move the lens, the appropriate conversion factor is read from the stored location and used to drive the lens to an appropriate position.
In the case in which the defocus amount is a small quantity, the relationship between the defocus amount and the lens drive amount is generally linear and the defocus amount d is converted into the lens drive amount L using a predetermined conversion factor k according to the following equation (1): EQU L=d/k (1)
For example, FIG. 14(a) shows a single lens 18 having a focal length f. A subject is at a distance x from the front side focal length. The distance from the rear side focal length to the subject image plane is y. From Newton's formula, the relationship of f, x and y is given by the following equation (2): EQU y=f.sup.2 /x (2)
In the case in which lens 18 is moved to the image plane side by an amount dx, as shown in FIG. 14(b), the movement of the image position must be considered. Differentiating equation (2), the amount of differential image plane movement dy is found from equation (3): EQU dy=(-f.sup.2 /x.sup.2).multidot.dx (3)
Because the actual amount of differential image plane movement dw is the difference between the image plane position of FIG. 14(a) and the image plane position of FIG. 14(b), dw can be represented by either of the following equations (4) or (5). ##EQU1## EQU dw=(1-y.sup.2 /f.sup.2).multidot.dx (5)
Equation (4) can be rewritten in the form of dw=k.multidot.dx where conversion factor k equals (1-f.sup.2 x.sup.2). Equation (5) can be written in the form dw=k.multidot.dx where conversion factor k equals (1-y.sup.2 /f.sup.2).
In equation (4), under normal photographic conditions, it can be assumed that x&gt;f [in equation (5), it can be assumed that f&gt;y]. Therefore, the term f.sup.2 /x.sup.2 becomes very small and the image plane movement amount dw becomes equal to the lens movement amount dx. In short, in the case of a single lens under normal photographic conditions, the conversion factor k of equation (1) is set to 1. Event in lens systems other than single lens systems, the lens movement amount and the image plane movement amount are linearly related when the subject is at a remote distance.
However, when the subject is at a close distance, the term f.sup.2 /x.sup.2 in equation (4) and the term y.sup.2 /f.sup.2 in equation (5) cannot be neglected. In this case, it is necessary to modify the conversion factor k according to equation (4) or equation (5).
In effect, when the defocus amount d is small and the image plane is moved by the defocus amount, equation (1) applies and the lens drive amount L is linearly related to the defocus amount d using the conversion factor k. However, when the defocus amount d is large, equations (4) and (5) apply.
From the application of equations (4) and (5), a phenomenon occurs in which the conversion factor changes and the lens drive amount L is no longer linearly related to the defocus amount d. In order to solve this problem, as shown in an autofocus adjustment apparatus disclosed in JP-A-62-170924 (Japanese Laid-Open Patent Publication 62-170924), the lens drive amount L is found in relation to the defocus amount d by introducing higher order terms. EQU L=d/{k0.multidot.(1+c0.multidot.d)} (6)
Here k0, c0 are predetermined constants. In an autofocus adjustment device with the lens drive amount L found by nonlinear conversion equation (6) above,; respective equations for the values of k0, c0 are established such that any error of the lens drive amount becomes small.
Focus adjustment systems, however, are typically only one part of a larger photographic optical system which includes various types of systems and subsystems. These systems and subsystems can include a whole group extension system, an inner focus system, a front lens extension system and a rear focus system. Therefore, it is difficult for one type of conversion equation (such as equation (6)) to cover all types of photographic optical systems. In some optical systems, even if the values of k0 and c0 are optimally established, the residual error becomes large and the focused state cannot be reached by driving the optical lens only one time. Therefore, it is usually required to drive the lens a number of times in order to properly focus a photographic optical system, thereby resulting in decreased responsiveness.